MPM Input Files

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MPM simulations are run using NairnMPM. Some of the key features of these code engines (with links) are listed here. The documentation on this page explains how to run MPM calculations by described all the sections of input command files.


Input Command File Overview

When using NairnFEAMPM or NairnFEAMPMViz to set up calculations, the input command files are usually script files written using the custom scripting language. When you interpret these scripts, the applications will create an XML input command file that is needed to drive the code engines. Click this link to see a sample scripted MPM input command file for simulation of two colliding disks.

Alternatively, you can directly create XML input command files. Because this approach cannot use the language control options of a scripting language, it is much less powerful then scripting methods and much harder to customize for a range of simulations. If you do use the manual XML method, the files must conform to the structure defined here. Click this link to see a sample XML MPM input command file for simulation of two colliding disks.

Main Header

This section defines the type of analysis to run and provides an optional description of the calculations. When using a scripting language, the header is created automatically and its content is controlled by commands defined below. When manually writing XML files, the header, which is required, is defined in a single header block:


The possible header commands are

MPM Header

This section defines many settings and features for the MPM calculations. When using a scripting language, the MPM header is created automatically by various commands defined in this section. When writing XML input files, all these commands must be within a single MPM header block:


The possible MPM header commands are given below by categories.

Archiving Results

MPM results can be saved as particle results or global results, where global results are averages over all particles, or specific material types during the calculations:

Besides the above built-in archiving options, some custom tasks provide additional options:

If more archiving options are needed, you can always write a new custom task.

Creating the MPM Background Grid

Although MPM is a particle method, it uses a background grid for calculations. In general, this grid is a regular grid of orthogonal elements with linear shape functions. The background grid is easily generated with a few commands. NairnMPM does allow explicitly generated grids, but you should never use them because many important features of MPM rely on a regular grid and are only efficient for a regular grid (e.g., uGIMP and CPDI shape functions).

Creating the Material Points

In MPM the object is discretized by a collection of material points. Once the background grid is created, the material points are added to that grid using a series of Region and Hole commands or additionally by directly translating pixels in an image to an MPM model. Each of these command fills part of the grid with a collection of material points and those material points are assigned initial conditions and material type.

Defining Material Models

Each material point is assigned a material type. NairnMPM supports numerous material models. For plasticity materials, each material type can choose from among a set of hardening laws. For softening materials, each material type can choose amoung a set of damage initiation laws and softening laws. When using explicit cracks, the material points along the cracks can be assigned to various traction laws. Finally when using multimaterial mode or using explicit cracks, the contact mechanics at that interfaces can be controlled by contact laws. The information on these options are in the following sections:

Using Explicit Cracks

An advanced feature of NairnMPM is that it can model explicit cracks including multiple cracks, interacting cracks, and crack propagation. To create a simultion with cracks, you first define the cracks and then optionally customize crack settings to customize crack modeling methods. To include crack propagation, you also need to set all needed material fracture properties. By choosing a default contact law (or by choosing custom contact laws for each crack), crack surfaces can model either frictional contact or model the crack plane as an imperfect interface.

Multimaterial MPM

Running in multimaterial mode allows a simulation to model contact between materials. By choosing a default contact law (or by choosing custom contact laws for each material pair), multimaterial mode simulations can model material interfaces as frictional contact or as an imperfect interface.

Boundary Conditions

In MPM, boundary conditions can be applied to nodes on the grid or applied directly on the particles:

  • Grid-Based Boundary Conditions - These boundary conditions are applied to nodes on the grid and are used to set velocity, temperature, and concentration. Grid-based velocity conditions can also set up object symmetry planes.
  • Particle-Based Boundary Conditions - These boundary conditions are applied to the particles and are used to set forces, tractions, and fluxes.

Thermal Calculations

MPM simulations can do thermal calculations that account for thermal strains in the stress analysis and can model thermal conduction.

Diffusion Calculations

MPM simulations can model diffusion and track concentration of an absorbed solvent in the materials. The concentration is couple to strains by solvent expansion coefficients of the materials.

Gravitation Field

MPM calculations can apply a gravitation field (or actually any constant body forces).

Custom Tasks

Custom tasks are special calculations that are performed at the end of each time step in MPM calculations. Custom tasks can be written by users working with NairnMPM source code. These user-defined custom tasks can then be scheduled when needed in some calculations. The current custom tasks are:

XML Insertion

When writing scripted input files, it is sometimes useful to insert raw XML commands into the automatically-format XML input file. The two commands that help in this task are:

  • Entity Command - to define entities that will appear in the automatically-format XML input file.
  • XMLData Command - to insert raw XML data into various sections of the automatically-format XML input file.
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